1. Field of the Invention
The present invention relates to electrically conductive microgel. More particularly, the present invention relates to electrically conductive microgel in which electrically conductive polymers or oligomers can be polymerized on the surface of microgel particles dispersed in an organic solvent, in the presence of a dopant and an oxidative polymerization catalyst, granting excellent conductivity to the microgel Also, the present invention is concerned with a method for preparing such electrically conductive microgel.
2. Description of the Prior Art
A number of applications of electrically conductive polymers for surface tension control, electromagnetic wave shielding, fuel batteries, etc. have been under study. Particularly, polyaniline, polypyrrole, and polythiophene are stable in air with high electrical conductivity. Another advantage of these conductive polymers is that they can be easily synthesized through electrochemical or chemical polymerization. However, films made of the electrically conductive polymers synthesized through electrochemical polymerization, although being constant in electrical conductivity across their surface areas, are greatly limited in uses because they are not easily melted owing to the strong intermolecular attractive force thereof. Powders of the electrically conductive polymers synthesized through chemical polymerization require complicated post-treatment processing for their applications.
Recently, extensive research has been directed to improving the processability of electrically conductive polymers. Preparation of water-soluble multi-component composite particles through chemical polymerization is an example of such research. Korean Pat. Laid-Open Publication No. 99-018821 discloses a method of preparing polyaniline and polypyrrole within ABS emulsion latex while using an organic acid such as dodecylbenzene sulfonic acid or paratoluene sulfonic acid as a dopant and ammonium persulfate as an oxidant. This method, however, is disadvantageous in that, when the monomers are polymerized, the presence of ion groups within the dopant and the emulsifier of the emulsion latex causes the monomers to be in an unstable water dispersion, resulting in undesired precipitation of powder. In turn, this precipitated powder is difficult to form into transparent films as well as requires additional homogenizing processes, such as pulverization and fine dispersion, in order that the powder can be coated on cloth or used in extrusion molding.
U.S. Pat. No. 6,001,549 describes an antistatic composition comprising electrically conductive polymer particles and copolymer microgel particles. The antistatic composition shows good conductivity at relatively low volume fraction of the electrically conductive particles by including microgel particles, which are prepared using an aqueous medium. However, the electrically conductive particles, selected from metal oxides, metal antimonates and ceramic particles, are unsuitable to give transparent or low anti reflective coating films.
U.S. Pat. No. 6,025,462 discloses an electrically conductive dendrimer whose reactive functional groups on its surface are useful to synthesize an electrically conductive polymer for composite particles which have been shown to solve the above problem encountered in the previous patent to some degree. However, the preparation process is too complicated to avoid imposing limitations on its commercialization.
Leading to the present invention, the intensive and thorough research on electrically conductive microgel, conducted by the present inventors, resulted in the finding that electrically conductive polymers or oligomers can be polymerized in a dispersion of microgel particles in the presence of a dopant and an oxidative polymerization catalyst and adsorbed onto the particle, granting excellent conductivity to the microgel particles.
Therefore, it is an object of the present invention to provide a method for preparing an electrically conductive microgel, by which excellent electrical conductivity can be obtained. It is another object of the present invention to provide an electrically conductive microgel which has a three-dimensional array of electrically conductive polymers in association with a polymeric binder, thereby showing excellent electrical conductivity, electromagnetic wave shielding, electrostatic prevention and film performance even though a small quantity of electrically conductive polymers are present therein.
In accordance with the present invention, there is provided a method for preparing an electrically conductive microgel comprising adding 3 to 30 weight % of a monomer for synthesizing electrically conductive polymer and 1 to 20 weight % of a dopant to 15 to 80 weight % of an organic solution containing 5 to 60 weight % of microgel particles and polymerizing said monomer at a temperature of 0 to 80xc2x0 C. under the addition of 2 to 40 weight % of an aqueous solution containing 1 to 40 weight % of an oxidative polymerization catalyst, in which the polymer adsorbed on the surface of the microgel particles.
In accordance with another object of the present invention, there is provided an electrically conductive microgel suitable for use in electrical conduction, electromagnetic-radio frequency interference shielding, electrostatic discharge protection and anti-fogging.
In the present invention, electrically conductive microgel is prepared by synthesizing electrically conductive polymers on the surface of microgel particles dispersed in an organic solvent and adsorbing the synthesized polymers thereonto. The microgel particles useful in the present invention are intramolecularly crosslinked, ultra-fine polymer particles with an average diameter of 0.01 to 10 microns and can be typically prepared by two processes. For reference, the term xe2x80x9can organic solution containing microgelxe2x80x9d or xe2x80x9ca microgel-containing organic solutionxe2x80x9d as used herein refers to a dispersion of microgel particles in an organic solvent.
One process for preparing a dispersion of microgel particles in an organic solvent is described in U.S. Pat. No. 4,403,003, which teaches a non-aqueous dispersion (AND) process. Also, a dispersion of microgel particles in an organic solvent can be obtained by a combination of emulsion polymerization and aqueous-organic solvent conversion, as taught in European Pat. Pub. No. 029637.
In the AND process, vinyl monomers are polymerized to microgel through dispersion polymerization in the presence of a polymer stabilizer in an organic solvent. With a steric stabilization ability, the polymer stabilizer acts to inhibit the flocculation of the microgel produced. In detail, the polymer stabilizer provides an energy barrier against particle flocculation by forming a chain extended configuration of the polymer around individual microgel particles. In the ""003 patent, the polymer stabilizer is prepared as follows. 12-Hydroxystearic acid is self-condensed to an acid value of about 31 to 34 mg KOH/g (corresponding to a number average molecular weight of 1650 to 1800) and then reacted with an equivalent amount of glycidyl methacrylate. The resulting unsaturated ester is copolymerized at a weight ratio of 2:1 with a mixture of methyl methacrylate and acrylic acid in the proportions of 95:5.
In another process, microgel can be prepared by the emulsion polymerization of vinyl monomers together with crosslinking monomers (e.g. divinyl monomers) in the presence of an emulsifier. Aqueous emulsions of microgel particles are being used industrially for an excellent metallic effect in paints for automobiles. In the present invention, however, a dispersion of microgel particles in an organic solvent is required, which can be obtained through the aqueous-organic solvent conversion process. This conversion process is known as a coagulation process as taught in WO 91/00895 and EP Publication No. 029638. When n-butanol is added to an aqueous microgel emulsion, coagulation of microgel particles occurs, resulting in the separation into two phases: a lower phase containing n-butanol, water and the emulsifier and an upper phase containing the microgel particles, n-butanol and the other components. The upper phase is azeotropically distilled in vacuo to remove residual water, thus obtaining a dispersion of microgel particles in an organic solvent.
Monomers suitable for use in forming the microgel particles used in the present invention are addition polymerizable monomers containing ethylenically unsaturated or more specifically vinylic, acrylic and/or allylic groups. Representative examples of the monomers include n-pentyl acrylate, n-butyl acrylate, benzyl acrylate, t-butyl methacrylate, 1,1-dihydroperfluorobutyl acrylate, benzyl methacrylate, chloromethylstyrene, ethyl methacrylate, isobutyl acrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, chloroprene, n-butyl methacrylate, isobutyl methacrylate, isopropyl methacrylate, lauryl acrylate, lauryl methacrylate, methyl acrylate, 2-ethoxyethyl acrylate, 2-ethoxyethyl methacrylate, 2-cyanoethyl acrylate, phenyl acrylate, isopropyl acrylate, n-propyl methacrylate, n-hexyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, styrene, vinylbenzyl acetate, vinyl benzoate and mixtures thereof.
Crosslinking monomers useful for forming the microgel particles used in the present invention are exemplified by allyl methacrylate, N,Nxe2x80x2-methylenebisacrylate, ethylene dimethacrylate, 2,2-dimethyl-1,3-propylene diacrylate, divinylbenzene, N,Nxe2x80x2-bis(methacryloyl)urea, 4,4xe2x80x2-isoproylidenediphenylene diacrylate, 1,3-butylene diacrylate, 1,4-cyclohexylenedimethylene dimethacrylate, ethylene diacrylate, 1,6-hexamethylene diacrylate, 1,6-diacrylamidohexane, 1,6-hexamethylene dimethacrylate, tetramethylene dimethacrylate, ethylenebis(oxyethylene) diacrylate, ethylenebis(oxyethylene)dimethacrylate, ethylenetrimethacrylate and 2-crotonoyloxyethyl methacrylate.
To be suitable for use in the present invention, the microgel particles have to range, in size, from about 0.01 to 10 microns and more preferably from about 0.01 to 1 micron. The microgel content in an organic solvent is preferably maintained in the range from 5 to 60 weight %. If the content of microgel particles is less than 5 weight %, the effects obtained by the addition thereof cannot be expected. On the other hand, a dispersion containing more than 60 weight % of microgel particles is too viscous to handle with ease.
In accordance with the present invention, the electrically conductive microgel is prepared by adding 3 to 30 weight % of monomers for synthesizing electrically conductive polymer and 1 to 20 weight % of dopants to 15 to 80 weight % of an organic solution containing 5 to 60 weight % of microgel particles and then polymerizing the monomers under the addition of 2 to 40 weight % of an aqueous solution containing 5 to 40 weight % of an oxidative polymerization catalyst. The synthesized polymer is adsorbed on the surface of the microgel particles. In the case of the monomers to be polymerized to an electrically conductive polymer being aniline, the reaction changes color from pale green to dark green during the polymerization. After completion of polymerization, phase separation occurs with a lower layer being an aqueous phase containing the oxidative polymerization catalyst and the lower layer can be removed with ease.
If the amount of the monomers is below 3 weight %, the resulting polymer is poor in conductivity. On the other hand, when the monomers are used at an amount more than 30 weight %, the microgel particles coagulate. Further, if an amount of the organic solution containing the microgel particles is less than 15 weight %, the microgel particles are coagulated to cause the problem of instability. On the other hand, the use of the solution exceeding 80 weight % suffers from too low conductivity. When the oxidative polymerization catalyst is used at an amount less than 2 weight %, the polymerization proceeds too slowly. On the other hand, in the presence of more than 40 weight % of the oxidative polymerization catalyst, the polymerization rate is too high to avoid the coagulation of the microgel particles.
The preparation of the electrically conductive microgel is preferably carried out in the range of 0 to 80xc2x0 C. The problem with the extra-range of the reaction temperature also resides in the polymerization rate. For example, if the reaction is carried out at less than 0xc2x0 C., the polymerization proceeds too slowly. On the other hand, at higher than 80xc2x0 C., the polymerization rate is so fast as to cause coagulation of the microgel particles. Depending on the concentration of the monomers and the polymerization temperature, it may take several seconds to several days to complete the polymerization.
With poor solubility in organic solvents, the electrically conductive polymer is adsorbed onto the surface of the microgel particles while being synthesized.
For the synthesis of the electrically conductive polymer in an organic solution containing the microgel particles are used monomers such as pyrrole, thiophene, indole, carbazole, furan, aniline, and derivatives thereof. For example, the pyrrole derivatives include N-methyl pyrrole, N-ethyl pyrrole, N-propyl pyrrole, and N-butyl pyrrole. Aniline derivatives useful in the present invention may be exemplified by N-alkyl aniline, 4-phenoxy aniline, 4-trimethylsilyl aniline, and 2,4-dimethoxy aniline 3,4-(alkylenedioxy)thiophene is also useful.
In the microgel particle-containing organic solution, the microgel particles are dispersed and stabilized through salvation by the hydrophobic organic solvent. The hydrophobic organic solvent is defined as being poorly water-compatible with a Hansen solubility parameter of 12.5 or less. Examples of the hydrophobic organic solvent available in the present invention include toluene, xylene, methylene chloride, chloroform, ethyl acetate and butyl acetate.
Oxidative polymerization catalysts that are generally used for preparing electrically conductive polymers are also used for the present invention. Their examples include ammonium persulfate, potassium persulfate, ferric chloride and ferric tosylate hydrogenperoxide.
To give rise to an increase in the conductivity of the microgel, the electrically conductive polymer is doped. Doping a conductive polymer entails chemically modifying the backbone to produce mobile charge carriers. With this purpose, commonly used dopants for preparing conductive polymers are usable in the present invention. Examples thereof include dodecylbenzenesulfonic acid, toluenesulfonic acid, camphorsulfonic acid, benzenesulfonic acid, hydrochloric acid, styrenesulfonic acid or 2-acrylamido-2-methylpropanesulfonic acid. The dopant is used in the mixture of the monomers for the synthesis of the electrically conductive polymer and preferably at an amount of 1 to 20 weight % based on the total weight of the whole solution (corresponding to an equivalent ratio of 0.3 to 1.5 to the monomers for the electrically conductive polymer). For instance, if the amount of the dopant is below 1 weight %, the polymer is doped insufficiently, resulting in low conductivity of the microgel. Over-doping with more than 20 weight % of the dopant also causes a decrease in conductivity of the electrically conductive microgel. According to the present invention, the electrically conductive microgel takes a core/shell structure in which electrically conductive polymers or oligomers are physically adsorbed onto internal crosslinked microgel particles.
For coating application, the electrically conductive microgel of the present invention may be combined with various binders, depending on the electrical conductivity and physical properties required for the coating. Associated with a binder, the microgel is maintained in a three dimensional structure when being dried (in the form of paint). Available as a binder in the present invention are those which are used in conventional paint. Suitable examples thereof include polyurethane resins, polyacrylic resins, thermosetting alkyd resins, and radiation curing vinyl monomers or oligomers. Preferably, the mixing ratic of the binder and the electrically conductive microgel of the present invention ranges 1:99 to 90:10 on the weight basis.
Optionally, a paint containing the electrically conductive microgel of the present invention may contain additives at an amount of up to 60 weight %. By way of example, not limitation, the additives include fillers and antioxidants. Useful as fillers are talc, barium sulfate, calcium carbonate, fibers, kaolin, pigment and conductive fillers. Additionally, in order to improve the performance of the electrically conductive film coated, an attaching enhancer, a thickener, a curing agent and/or an organic solvent are useful.
Organic solvents can be removed in a freeze drying process, a spray drying process or a vacuum drying process. To be useful as a solid material, the microgel of the present invention can be treated in an ordinary process (e. g., injection or extrusion molding). The electrically conductive materials or composite materials of the present invention can find applications in the electromagnetic-radio frequency interference shielding, antistatic, and anti-fog industries.